Geological Observations of Volcanic Islands | Page 8

in his book on South American geology.
The proofs of recent elevation around many of the volcanic islands led
Darwin to conclude that volcanic areas were, as a rule, regions in which
upward movements were taking place, and he was naturally led to
contrast them with the areas in which, as he showed, the occurrence of
atolls, encircling reefs, and barrier-reefs afford indication of subsidence.
In this way he was able to map out the oceanic areas in different zones,
along which opposite kinds of movement were taking place. His
conclusions on this subject were full of novelty and suggestiveness.
Very clearly did Darwin recognise the importance of the fact that most
of the oceanic islands appear to be of volcanic origin, though he was
careful to point out the remarkable exceptions which somewhat
invalidate the generalisation. In his "Origin of Species" he has
elaborated the idea and suggested the theory of the permanence of
ocean-basins, a suggestion which has been adopted and pushed farther
by subsequent authors, than we think its originator would have
approved. His caution and fairness of mind on this and similar
speculative questions was well-known to all who were in the habit of
discussing them with him.
Some years before the voyage of the "Beagle," Mr. Poulett Scrope had
pointed out the remarkable analogies that exist between certain igneous
rocks of banded structure, as seen in the Ponza Islands, and the foliated
crystalline schists. It does not appear that Darwin was acquainted with
this remarkable memoir, but quite independently he called attention to
the same phenomena when he came to study some very similar rocks
which occur in the island of Ascension. Coming fresh from the study of

the great masses of crystalline schist in the South American continent,
he was struck by the circumstance that in the undoubtedly igneous
rocks of Ascension we find a similar separation of the constituent
minerals along parallel "folia." These observations led Darwin to the
same conclusion as that arrived at some time before by Scrope--namely
that when crystallisation takes place in rock masses under the influence
of great deforming stresses, a separation and parallel arrangement of
the constituent minerals will result. This is a process which is now fully
recognised as having been a potent factor in the production of the
metamorphic rock, and has been called by more recent writers
"dynamo-metamorphism."
In this, and in many similar discussions, in which exact mineralogical
knowledge was required, it is remarkable how successful Darwin was
in making out the true facts with regard to the rocks he studied by the
simple aid of a penknife and pocket-lens, supplemented by a few
chemical tests and the constant use of the blowpipe. Since his day, the
method of study of rocks by thin sections under the microscope has
been devised, and has become a most efficient aid in all petrographical
inquiries. During the voyage of H.M.S. "Challenger," many of the
islands studied by Darwin have been revisited and their rocks collected.
The results of their study by one of the greatest masters of the science
of micropetrography--Professor Renard of Brussels--have been recently
published in one of the volumes of "Reports on the 'Challenger'
Expedition." While much that is new and valuable has been contributed
to geological science by these more recent investigations, and many
changes have been made in nomenclature and other points of detail, it
is interesting to find that all the chief facts described by Darwin and his
friend Professor Miller have stood the test of time and further study,
and remain as a monument of the acumen and accuracy in minute
observation of these pioneers in geological research.
JOHN W. JUDD.
CHAPTER I.
--ST. JAGO, IN THE CAPE DE VERDE ARCHIPELAGO.

Rocks of the lowest series. A calcareous sedimentary deposit, with
recent shells, altered by the contact of superincumbent lava, its
horizontality and extent. Subsequent volcanic eruptions, associated
with calcareous matter in an earthy and fibrous form, and often
enclosed within the separate cells of the scoriae. Ancient and
obliterated orifices of eruption of small size. Difficulty of tracing over a
bare plain recent streams of lava. Inland hills of more ancient volcanic
rock. Decomposed olivine in large masses. Feldspathic rocks beneath
the upper crystalline basaltic strata. Uniform structure and form of the
more ancient volcanic hills. Form of the valleys near the coast.
Conglomerate now forming on the sea beach.
(FIGURE 1: MAP 1: PART OF ST. JAGO, ONE OF THE CAPE DE
VERDE ISLANDS.)
The island of St. Jago extends in a N.N.W. and S.S.E. direction, thirty
miles in length by about twelve in breadth. My observations, made
during two visits, were confined to the southern portion within the
distance of a few leagues from Porto Praya. The country, viewed from
the sea, presents a varied outline: smooth conical